In a previous post of mine I briefly introduced a beautiful example of blending science with performing arts: that was "Gravity: the dance of space and time", an initiative in which I took part recently at the University of Maryland, in collaboration with the School of Dance Instructor Adriane Fang and Astronomy Professor Cole Miller.
In the present post I would like to present this performance in more detail, by pointing at the background scientific concepts and how they got translated artistically into the show, which you can see in its final form at this link.
Inspired by the "Dance your PhD" contest, Adriane was interested in bringing some science into dance and that's where Cole and I came to the rescue: though we did not end up dancing in the show as it is required to the contest participants, we got actively involved in the rehearsals, not only building a conversation with the artists but also trying some moves out. I hope my following description will convey the feelings of emotion and satisfaction that I experienced during all the stages of the project.
Even though gravity might sound like something obvious and a completely figured out concept it is actually among the most intriguing domains of current investigations in both theoretical and experimental physics. Just think about the mysterious dark matter and dark energy and the fact that they account for as much as 96% of the total mass-energy density of the universe. In our everyday life we only have one chance to appreciate how gravity is far from evident, when we use the GPS antenna in our navigator or smartphone: if Einstein had not improved on Newton's grasp of gravity the GPS could not exist or work. In Newton's description gravity is a force that propagates instantaneously, for example from the Sun to the Earth: if one could make the Sun disappear we would immediately realize the absence of its gravitational pull on Earth (see for example a video from Brian Greene's documentary "The Elegant Universe"- Episode 1, 9:30 into it). The set in which this happens is as static as a fixed stage, where every actor experiences things in the same way, most notably for what concerns time. Then came Einstein. In his picture gravity is still due to the presence of mass but there is something more profound to it: mass deforms space in a way similar to how a heavy ball acts on a trampoline or to when we sit on a couch pillow; objects put in the vicinity of the deformation fall towards the mass responsible for that, just as we see them falling toward the Earth when we release them to the pull of its gravity. What does this have to do with GPS? The answer lies in the fact that, with Einstein, space is no longer a static stage with one given universal time: there exists a single entity called spacetime, which is a dynamic stage that can do stuff and participates to the acting.
When our GPS antenna talks to the GPS satellite fleet to establish its position relative to the satellites', an exchange of signals is involved in the process; the situation is reminiscent of clock synchronization among people: if everyone's watch shows a different time there are very few chances to recombine all together on time. In the case of GPS satellites communicating to our antenna, synchronization is not so easy: for starters time does not flow at the same pace for everyone! that's what a dynamical spacetime stage entails. If mass can deform space, and space is a whole with time, mass affects time: the closer you are to the source of deformation, the slower time flows for your watch as compared to one which is at a larger distant from the mass. Finally, there's one more source of difference between the pace of satellites' time and the one of clocks on Earth's surface, speed effects: the faster you move the slower time flows for your watch as compared to one which is at rest. It wouldn't be worthy of Einstein if things were not so rich!

I find Salvador Dali's "The Persistence of Memory" a powerful visual handle to grasp the concept of mutable time.

This was kind of a long introduction but it will allow you to better appreciate the dance show, especially its second part: in fact, while the first act represents the motion of astrophysical objects in spacetime, the second is devoted to spacetime itself. For this reason, I'm going to talk about the final half of the show first.
In collaboration with costume designer Kate Fulop, we chose black stretchy costumes to be used in the second act: they were meant to represent spacetime as an elastic deformable cosmic fabric. The moves the dancers perform are both artistically pleasant and scientifically suggestive: they alternate between slow and fast, just as we said time can flow in a specific region of space according to the proximity of this region to a heavy astrophysical mass.

Of course, we did not want the dance performance to be just descriptive: that's what I meant earlier on when I said that the entire collaboration has been the result of a conversation around a scientific theme. Adriane proposed her graduate students to perform their moves according to an interesting interpretation of the scientific concepts: in pairs, the artists would stimulate their partner's movement by transmitting them their own energy through a flow without contact; then the partners would react either by affinity or contrast, that is to say moving towards or against the source of energy, respectively. I personally took part in the rehearsals in which the dancers were exploring this part of their "phrase", as it is called in their jargon: for me it was both new and challenging to try and bring formulae alive in this way. Another distinctive type of the grad students' moves inspired by science was the "stretch and squeeze". In order to explain it let me go back to the trampoline analogy I used to depict how spacetime gets deformed in the presence of mass. Imagine moving the mass around on the surface of the trampoline: you can picture ripples forming on the elastic membrane, just like waves on the surface of a pond. This might make you think of yet another type of waves coming from a perturbed membrane: the ones coming from a drum hit by mallets, that is to say sound waves. Like a buoy is carried up and down by the tide a device probing spacetime ripples would experience two peculiar effects: the aforementioned stretch and squeeze.
The sound you hear at the end of the first act is the melody played by two huge cosmic mallets hitting on the spacetime drum, a couple of merging black holes. This is the result of a simulation where the astrophysical signal expected from the coalescence has been treated in such a way as to shift its frequency to the region audible to our ears: in fact, these gravitational waves do not bring any type of light by themselves, so we will not "see" them but rather "listen" to them with our instruments. Given the variety of astrophysical sources and configurations, scientists expect to listen to a sort of very peculiar concert of gravitational waves: in the next few years instruments will be upgraded to the necessary sensitivity and we could hear as many as a hundred of different "music pieces" per year.
On scene the sound simulation accompanies the evolutions of the last two dancers in the first act: they represent two black holes orbiting around each other in a spiraling shrinking motion dictated by Einstein's equations; the very last stage of the evolution, the merger of the two bodies, is described by the powerful moment of a hug between the two dancers. One of them is still carrying her veil. This element of the costumes is instrumental to the science too. When an astrophysical object passes by another its companion experiences a varying gravitational field, thus the companion deforms its shape. This is fancy talk to refer to Earth's tides; due to the varying distance of the Moon our planet gets periodically deformed on two sides: the one closer to the Moon, which is feeling its gravitational pull more strongly, and the other farther from the Moon, which is feeling its gravitational pull less strongly. At a more quantitative level such tidal deformations, and their physical effects, are nicely represented by simulations such as this one from Caltech.

Dancers playing star encounters in the universe.(Copyright Stan Barouh http://stanbarouhphotography.smugmug.com/Theater/University-of-Maryland-School)

The first act of the performance is then a joyful succession of star and black hole encounters, something that cannot happen in our astronomical neighborhood because it is not very populated. While this is good for the survival of the human race on Earth it is kind of boring for the curious scientists. Soon they will be able to add yet more information to their comprehension of astronomy by opening a new observation window on the Universe: this is what scientists such as Cole and I call gravitational wave astronomy; together with Adriane, her amazing students and her friendly colleagues we happily participated in building a representation of the subject that could be attractive to non-scientists. We hope we succeeded. Now watch the video of the performance again and see if you think likewise.

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